Processing servo data having DC level shifts

ABSTRACT

A read channel component of a magnetic recording system employs equalization of a signal received from the magnetic recording channel, the equalization being modified depending upon the presence or absence of DC shifts in the signal. Equalization corrects for DC shifts, if present, prior to detection and decoding of servo data, such as servo address mark (SAM) and Gray code data. In a first implementation, a DC shift detector detects the presence or absence of DC shifts and modifies equalization in a predetermined manner. In a second implementation, filtering is applied to the signal to enhance equalization in the presence of DC shift, and both filtered and unfiltered signals employed for detection of the servo data.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of the filing date of U.S.provisional application No. 60/457,041, filed on Mar. 24, 2003 asattorney docket no. 992.1094PROV.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to detection of data in acommunications system, and, more particularly, to processing of servodata information read from a channel.

[0004] 2. Description of the Related Art

[0005] A read channel integrated circuit (IC) is a component of a moderndisk drive, such as a hard disk drive found in many PCs. A read channelcomponent converts and encodes data to enable the (e.g., magnetic)recording head(s) to write data to the disk and then read back the dataaccurately. The disks in a hard disk drive typically include many trackscontaining encoded data, and each track comprises one or more of user(or “read”) data sectors as well as “servo” data sectors embeddedbetween the read sectors. The information of the servo sectors aids inpositioning the magnetic recording head over a track on the disk so thatthe information stored in the read sectors may be retrieved accurately.

[0006]FIG. 1 shows a conventional magnetic recording system 100 of theprior art. Servo information is encoded by block encoder 101, and blockencoder 101 may represent one or more different encoders associated withdifferent fields of the servo information, such as Gray code and servoaddress mark (SAM) data. The encoded servo information is written to thedisk (or other recording medium) as servo sector information.

[0007]FIG. 2 shows the format of servo sector information 200. Servosector information 200 comprises preamble 201 (e.g., a 2T pattern) thatallows the system to recover the timing and gain of the written servodata. Preamble 201 may be followed by encoded SAM data 202, which isgenerally an identical identification address (fixed number of bits) forall servo sectors. SAM data 202 may then be followed by Gray data 203(i.e., encoded Gray code). Gray data 203 represents tracknumber/cylinder information and may be employed as coarse positioninginformation for the magnetic head. One or more burst demodulation fields204 follow Gray data 203. Burst demodulation fields 204 are employed asfine positioning information for the head over the track.Repeatable-run-out (RRO) data field 205 follows burst demodulationfields 204. RRO data in RRO data field 205 provides head-positioninginformation to correct for RRO, which occurs when the head does nottrack an ideal path over the disk. RRO information is finer than thatprovided by the Gray data and coarser than that provided by the burstdemodulation fields.

[0008] Returning to FIG. 1, the encoded servo information is read backby a magnetic recording head. Together, the process of writing to,storing on, and reading from the disk by the recording head may bemodeled as magnetic recording channel 102 with added noise and DCshifts. Data read from the disk is referred to as readback data. Thereadback data is equalized to a desired target partial response byequalizer 103. Equalizer 103 comprises continuous time filter (CTF) 120followed by discrete time, finite impulse response (FIR) filter 121.Sampling of the signal from CTF 120 might be accomplished via switch122. Sampling might be synchronous using the timing information fromdigital phase locked loop (DPLL) 123 when servo SAM, Gray, anddemodulation burst data are read, but might also be asynchronous if DPLL123 is not used. Sampling of the signal from CTF 120 might beasynchronous when RRO data is read. The output of equalizer 103 isdigitized and quantized by analog-to-digital converter (ADC) 104, whoseoutput is shown as Y values.

[0009] For either synchronous and asynchronous sampling, the Y valuesmight be applied to data detector 105, which is typically apartial-response maximum-likelihood (PRML) detector employing, forexample, a Viterbi algorithm. Detector 105 may also be implemented witha slicer. Constraints imposed by the servo-encoding algorithm of blockencoder 101 might be employed in the design of data detector 105 foroptimal decoding of the encoded servo information. The output of datadetector 105 is applied to SAM detector 107 to detect the SAM data. Theoutput of data detector 105 and the output of SAM detector 107 areapplied to Gray code decoder 108 to generate decoded Gray data. The ‘Y’values are also applied to burst demodulator 111 to generate finepositioning information for the head over the track.

[0010] For asynchronous sampling, such as when DPLL 123 is not used orwhen reading RRO data, data phase generator 109 and data phase selector110 might be employed. Data phase generator 109 generates one or moreadditional sample sequences from the Y values, each additional samplesequence having a different phase relative to the phase of asynchronoussamples from ADC 104. The one or more additional sample sequences mightbe generated either by asynchronous over-sampling or by interpolation ofthe asynchronous samples from ADC 104. The one or more additional samplesequences and the asynchronous samples from ADC 104 are provided to dataphase selector 110. Data phase selector 10 selects of the inputsequences for use by data detector 105 based on a determination of whichsequence phase is closest to those having ideal timing.

[0011] In addition to noise, DC (baseline level) shifts might impair thesignal of recording channel 102. Performance of magnetic recordingsystem 100, as measured by SAM detection error rate and Gray codedetection error rate, might be degraded considerably when largeamplitude DC shifts corrupt the encoded servo information signal. TheseDC shifts might occur when the read head becomes unstable. FIG. 3A showsa graph of waveforms with DC baseline shift (shown as dashed lines) andwithout DC baseline shift (shown as solid lines) before sampling, andFIG. 3B shows a graph of waveforms with and without DC baseline shiftafter equalization and sampling (circles are sample points). Shown inFIGS. 3A and 3B are the input servo signal as well as one phase of theservo signal at the output of ADC 104 before data detection,respectively.

[0012] Data detector 105 might detect positive and negative peaks in theservo signal, but DC shifts in the servo signal cause severe signaldiscontinuities. DC shifts might occur i) randomly within the servosignal, ii) with random duration, and iii) multiple times. Consequently,DC shifts of magnetic recording system 100 are different from a fixed DCoffset applied to the entire servo signal or a fixed offset applied tothe signal corresponding to individual servo-encoded words. Depending onwhere the DC shifts occur in the signal, the DC shift might lead to asevere reduction in amplitude of the peaks in the signal, preventingreliable data detection regardless of the type of data detectionemployed by system 100. For example, in FIGS. 3A and 3B, the amplitudeof the negative peak around time 1025 is severely degraded. Lessreliable data detection results in an increase in the SAM detection andGray bit error rates, which inhibits proper operation of the servosystem and, in particular, the throughput of the servo system.

SUMMARY OF THE INVENTION

[0013] The present invention relates to equalization of a signalreceived from a channel in which the equalization is modified dependingupon the presence or absence of DC shifts in the signal prior todetection of, for example, servo data. In one case, the presence orabsence of DC shifts is detected and equalization is modified in apredetermined manner. In another case, filtering is applied to thesignal to enhance equalization in the presence of DC shift, and bothfiltered and unfiltered signals are employed for detection of the servodata.

[0014] In accordance with exemplary embodiments of the presentinvention, data in a signal read from a channel is detected by applying,with an equalizer, equalization to the signal to account for a DC shiftin the signal; and detecting the data based on either the presence orabsence of the DC shift in the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Other aspects, features, and advantages of the present inventionwill become more fully apparent from the following detailed description,the appended claims, and the accompanying drawings in which:

[0016]FIG. 1 shows a conventional magnetic recording system of the priorart;

[0017]FIG. 2 shows a fornat for servo sector information employed withthe magnetic recording system of FIG. 2;

[0018]FIG. 3A shows a graph of waveforms with and without DC baselineshift;

[0019]FIG. 3B shows a graph of waveforms with and without DC baselineshift after sampling;

[0020]FIG. 4 shows a receiver for detecting and decoding data accountingfor DC shift in accordance with a first exemplary embodiment of thepresent invention;

[0021]FIG. 5 shows a receiver for detecting and decoding data accountingfor DC shift in accordance with a second exemplary embodiment of thepresent invention;

[0022]FIG. 6 shows an exemplary method of conditioning employed by theSAM detector and Gray code decoder of FIGS. 5; and

[0023]FIG. 7 shows an exemplary implementation of the digital postfilter equalizer of FIGS. 5 and 6.

DETAILED DESCRIPTION

[0024] In accordance with exemplary embodiments of the presentinvention, equalization of a signal received from a channel is modifieddepending upon the presence or absence of DC shifts in the signal priorto detection and decoding of, for example, servo data. Equalizationmight be modified by first detecting DC shifts, or filtering of thesignal may be employed to enhance equalization in the presence of DCshifts. When filtering is employed, both filtered and unfiltered signalsmight be employed for detection and decoding of the servo data.

[0025]FIG. 4 shows a receiver 400 for detecting and decoding servo dataaccounting for DC shifts in accordance with a first exemplary embodimentof the present invention. Receiver 400 comprises equalizer 401 havingcontinuous time filter (CTF) 402 and finite impulse response filter 403,analog-to-digital converter (ADC) 404, and DC shift detector 405.Receiver 400 further comprises burst demodulator 411, data phasegenerator 406, data phase selector 407, data detector 408, servo addressmark (SAM) detector 409, and Gray code decoder 410.

[0026] Receiver 400 receives an analog signal read from, for example, amagnetic recording channel. The analog signal may represent encodedservo information, such as encoded Gray and SAM data read from a disk(or other recording medium) as servo sector information. The analogsignal is equalized to a desired target partial channel response, suchas an EPR4 ([5 5 −5 −5]) response. Equalization by equalizer 401 tendsto correct for effects of inter-symbol interference resulting from thesignal passing through the magnetic recording channel. FIR filter 403may be characterized by a set of filter tap coefficients. Methods fordetermining both analog filter circuit components for CTF 402 and filtertaps for FIR filter 403 to equalize the input signal are well known inthe art. In addition, equalizer 401 may correct for DC shifts in theinput signal.

[0027] Sampling of the signal from CTF 402, shown in FIG. 4 by switch422, is asynchronous and the output of equalizer 401 is digitized andquantized by ADC 404, whose output is an asynchronous sample sequenceshown as ‘Y’ values. The asynchronous sample sequence from ADC 404 isapplied to DC shift detector 405. DC shift detector 405 detects DCshifts in the sampled, equalized signal from the recording channel. DCshift detector 405 may be implemented by, for example, a threshold orsimilar level detector. As described subsequently, when a DC shift isdetected, DC shift detector 405 provides a signal to equalizer 401 tomodify operation of equalizer 401 to correct for the detected DC shift.

[0028] The asynchronous sample sequence is applied to data phasegenerator 406. Data phase generator 406 generates one or more additionalsample sequences from the Y values, each sample sequence having a phaserelative to the asynchronous samples from ADC 404. The one or moreadditional sample sequences might be generated either by asynchronousover-sampling or by interpolation of the asynchronous samples from ADC404. The one or more additional sample sequences of data phase generator406 and the asynchronous samples from ADC 404 are provided to data phaseselector 407. Data phase selector 407 selects one (or possibly two) ofthe input sequences for use by data detector 408 based on adetermination of which sequence phase is closest to the ideal samplesequence. The initial data phase is typically determined during apreamble or 2T pattern within the encoded servo data, and thencontinually updated thereafter.

[0029] The sample sequence selected by data phase selector 407 isapplied to data detector 408, which is typically a partial-responsemaximum-likelihood (PRML) detector employing, for example, a version ofthe well-known Viterbi algorithm. Data detector 408 might also beimplemented as a slicer. Constraints imposed by the algorithm employedto encode the servo data might be employed in the design of datadetector 408 for optimal decoding of the encoded servo information. Theoutput of data detector 408 is applied to SAM detector 409 to detect theSAM data. The output of data detector 408 and the output of SAM detector409 are applied to Gray code decoder 410 to generate decoded Gray data.Methods for detection of SAM data and decoding of Gray data are wellknown in the art. The ‘Y’ values are also applied to burst demodulator411 to generate fine positioning information for the head over thetrack.

[0030] Operation of DC shift detector 405 is now described. Examinationof analog waveforms representing servo data with DC shifts shows thatthe signal peaks present in a servo signal might be enhanced using anequalizer that provides gain or boost at higher frequencies above thosecorresponding to the DC shifts up to and beyond the Nyquist frequency.Thus, the corner frequency of the filter for CTF 402 might be set to theNyquist frequency, providing Nyquist equalization. Nyquist equalizationmight degrade performance of receiver 400 with respect to SAM detectionand Gray code decoding when only noise, and no DC shifts, are present asan impairment (i.e., when the head is in a “stable” state). The readhead might transition from a stable state to a long period ofinstability, or unstable state, over multiple servo sectors beforereturning to the stable state.

[0031] Consequently, the presence of DC shifts are detected by DC shiftdetector 405, which generates a signal to change equalization of CTF 402to Nyquist equalization during periods having DC shifts. Although agiven implementation of DC shift detector might take approximately oneservo sector processing time period to detect a DC shift and changeequalization by equalizer 401, each subsequently processed sectorbenefits from the Nyquist equalization. Once DC shift detector 405detects that a DC shift is no longer present, DC shift detector 405generates a signal to switch equalization by equalizer 401 back to theoriginal setting.

[0032] The first exemplary embodiment of FIG. 4 preferably employs adigital realization of DC shift detector 405, while otherimplementations might employ an analog DC shift detector instead. Whileadditional analog circuitry is employed when the analog DC shiftdetector is used, an analog DC shift detector might allow for fasterdetection of DC shifts.

[0033] An implementation for DC shift detector 405 might employ one of anumber of different methods known in the art. For example, oneimplementation observes a moving average output of ADC 404 and declaresthe presence of multiple DC shifts due to an unstable head when themoving average crosses a threshold. The moving average might be computedover a predetermined number W of samples, and the corresponding movingaverage filter (having filter length W) has W “1”s as its impulseresponse. The threshold might be optimized to minimize false DC shiftdetection based on a cost criterion. A related implementation for DCshift detector 405 might declare the absence or presence of DC shifts byobserving the average output of ADC 404 over a servo sector. For thecase where head instability is sporadic over a sector, but tends torepeat from sector to sector, DC shift detector 405 might declare thepresence of DC shifts when the moving average output of ADC 404 crossesthe threshold a given number of times during the sector. Similarly, theabsence of DC shifts is declared when the moving average output does notcross the threshold during a sector. Yet another implementation for DCshift detector 405 might observe the absolute value of the output of ADC404 over time to detect changes in baseline DC level.

[0034]FIG. 5 shows a receiver 500 for detecting and decoding dataaccounting for DC shift in accordance with a second exemplary embodimentof the present invention. Receiver 500 comprises equalizer 501 havingcontinuous time filter (CTF) 502 and finite impulse response filter 503,and analog to digital converter (ADC) 504. Receiver 500 furthercomprises burst demodulator 512, data phase generator 505, data phaseselector 506, post filter equalizer 508, data detectors 507 and 509, SAMdetector 510, and Gray code decoder 511.

[0035] Equalizer 501 and ADC 504 each operate in a similar manner tothat of equalizer 401 and ADC 404, respectively, of FIG. 4, except thatequalization of equalizer 501 is not switched to Nyquist equalization inthe presence of DC shifts. In addition, data phase generator 505 anddata phase selector 506 each operate in a similar manner to that of dataphase generator 406 and data phase selector 407, respectively, of FIG.4. Burst demodulator 512 receives unfiltered values from ADC 504.

[0036] The second exemplary embodiment differs from the first exemplaryembodiment in that a DC shift detector is not employed; instead, postfilter equalizer 508 is employed to post-process the sample sequencefrom data phase selector 506 for improved performance of SAM detector510 and Gray code decoder 511. If the DC shifts occur with relativelygreat frequency, then more than a full servo sector might be required todetect the presence of DC shifts. The second exemplary embodimentcorrects for effects of DC shifts by using a digital realization ofeither a finite impulse response (FIR) or an infinite impulse response(IIR) filter to post-process the sample sequence from data phaseselector 506. The coefficients of digital post filter equalizer 508might either be programmable or adaptively set based on an errorcriterion, such as minimum squared or absolute Euclidean distance. Thefilter coefficients of digital post filter equalizer 508 are in generalset to boost high-frequency signal components near the Nyquist frequencyin a manner similar to that described previously.

[0037] When no DC shifts are present, performance of SAM detector 510and Gray code decoder 511 might not be as good if post filter equalizer508 first processes a detected sample sequence. Consequently, the samplesequence from data phase selector 506 is provided to two paths. In onepath, the sample sequence is simply provided to data detector 507, whilein the other path the sample sequence is first processed by post filterequalizer 508 and then applied to data detector 509. Data detectors 507and 509 each operate in a manner similar to that of data detector 408 ofFIG. 4, and data detectors 507 and 509 may be equivalent. Alternatively,data detectors 507 and 509 may be optimized to account for whether ornot the detector processes a sample sequence from post filter equalizer508.

[0038]FIG. 6 shows an exemplary configuration for conditioning employedby the SAM detector and Gray code decoder of FIG. 5. SAM detector 510 ofFIG. 5 comprises conditioning logic 601 and SAM detection logic 603(a)and 603(b). SAM conditioning logic 601 generates a logic OR of theoutputs of SAM detection logic 603(a) and (603(b). SAM detection logic603(a) detects the SAM based on the unfiltered path output of datadetector 507, while SAM detection logic 603(b) detects the SAM based onthe filtered path output of data detector 509. In one implementation,Gray code decoder 511 includes Gray code decoding logic 604, whichgenerates the Gray code output based on only the output of data detector507 (the unfiltered, selected sample sequence). Conditioning logic 601might be modified from the simple OR gate.

[0039] In another implementation, Gray code decoder 511 includesdecision logic 602 in addition to Gray code decoding logic 604. Decisionlogic 602 might either i) combine the information from the two filteredand unfiltered path streams or ii) select one of the two streams fromdata detectors 507 and 509. Decision logic 602 of FIG. 6 might beconfigured to introduce a quality metric for use in selecting which bitto provide as input to Gray code decoding logic 604. One example of aquality metric might include a determination made for the outputs of thetwo data detectors (e.g., data detectors 507 and 509) that, for a givenpeak location corresponding to a particular bit, which data detectorproduced the larger peak.

[0040]FIG. 7 shows an exemplary implementation 700 of digital postfilter equalizer 508 of FIGS. 5 and 6. Post filter equalizer 700 is a(2-D) FIR filter (“D” stands for one sample discrete time delay)comprising multipliers 701 and 702, flip-flops 703 and 704, and adder705. The current sample multiplied by coefficient (2) in multiplier 702is added in adder 705 to the previous sample i) multiplied by −1 inmultiplier 701 and ii) stored in flip-flop 703. The result from adder705 is stored in flip-flop 704. As would be apparent to one skilled inthe art, other filters may be employed for a post filter equalizer 508.

[0041] For the described second exemplary embodiment, the data phaseupdate by data phase generator 505 and data phase selector 506 isperformed based on the unfiltered sample stream from ADC 504 only.However, for some implementations, the data phase selection and updateprocess might occur for the filtered path instead of the unfilteredpath. Consequently, post filter equalizer 508 may be placed prior todata phase generator 505 and data phase selector 506 in the signal path.

[0042] While the present invention is described employing an EPR4 ([5 5−5 −5]) target partial channel response, the present invention is not solimited. One skilled in the art may extend the teachings herein todifferent target partial channel responses. While the present inventionis described for detection and decoding of encoded servo data from amagnetic recording medium, the present invention is not so limited. Oneskilled in the art may readily extend the teachings herein to sampleddata read from other types of recording media, such as optical recordingmedia.

[0043] A receiver employing one or more embodiments of the presentinvention may have substantially improved detection performance forreadback data. Implementations of the one or more embodiments mightexhibit such improved detection performance for readback data withoutdegrading the detection performance of the servo demodulation data orthe SAM and Gray data when no DC shifts are present to corrupt the servosignal. Such improved detection performance might improve the SAMdetection and Gray bit error rate performance of a system in thepresence of DC shifts by as much as an order of magnitude or more.

[0044] The present invention can be embodied in the form of methods andapparatuses for practicing those methods. The present invention can alsobe embodied in the form of program code embodied in tangible media, suchas floppy diskettes, CD-ROMs, hard drives, or any other machine-readablestorage medium, wherein, when the program code is loaded into andexecuted by a machine, such as a computer, the machine becomes anapparatus for practicing the invention. The present invention can alsobe embodied in the form of program code, for example, whether stored ina storage medium, loaded into and/or executed by a machine, ortransmitted over some transmission medium, such as over electricalwiring or cabling, through fiber optics, or via electromagneticradiation, wherein, when the program code is loaded into and executed bya machine, such as a computer, the machine becomes an apparatus forpracticing the invention. When implemented on a general-purposeprocessor, the program code segments combine with the processor toprovide a unique device that operates analogously to specific logiccircuits.

[0045] It will be further understood that various changes in thedetails, materials, and arrangements of the parts which have beendescribed and illustrated in order to explain the nature of thisinvention may be made by those skilled in the art without departing fromthe principle and scope of the invention as expressed in the followingclaims.

What is claimed is:
 1. An apparatus for detecting data in a signal read from a channel, the apparatus comprising: an equalizer configured to apply equalization to the signal to account for a DC shift in the signal; and a detector adapted to detect the data based on either the presence or absence of the DC shift in the signal.
 2. The invention as recited in claim 1, further comprising: a DC shift detector adapted to detect the presence or absence of the DC shift in the signal and to generate a corresponding DC shift detect signal; and wherein the equalizer, in response to the DC shift detect signal, modifies the equalization based on the detected presence or absence of the DC shift in the signal.
 3. The invention as recited in claim 2, wherein the equalizer modifies the equalization to Nyquist equalization when the DC shift detect signal indicates the presence of the DC shift in the signal.
 4. The invention as recited in claim 1, wherein the equalizer is a post filter equalizer, and the apparatus further comprises: a combiner adapter to combine i) the signal before applying the equalization and ii) the signal after applying the equalization into a combined signal; and a detector adapted to employ conditioned detection of the data based on the combined signal.
 5. The invention as recited in claim 4, wherein the combiner is a logic OR.
 6. The invention as recited in claim 4, wherein the data is servo data comprising servo address mark (SAM) data and Gray data, and wherein the detector detects the SAM data based on the combined signal.
 7. The invention as recited in claim 6, wherein the apparatus further comprises a decoder adapted to decode the Gray data in the signal without applying the equalization based on the detected SAM data.
 8. The invention as recited in claim 6, wherein the apparatus further comprises decision logic adapted to select either the i) the signal before applying the equalization and ii) the signal after applying the equalization for decoding the Gray data.
 9. The invention as recited in claim 8, wherein the decision logic selects based on a detection metric.
 10. The invention as recited in claim 4, wherein the post filter equalizer is a 2-D filter, where “D” is a unit discrete time delay.
 11. The invention as recited in claim 1, wherein the apparatus is embodied in an integrated circuit (IC).
 12. The invention as recited in clain 1, wherein the IC is implemented in a read channel component of either a magnetic recording system or an optical recording system.
 13. A method of detecting data in a signal read from a channel comprising the steps of: (a) applying, with an equalizer, equalization to the signal to account for a DC shift in the signal; and (b) detecting the data based on either the presence or absence of the DC shift in the signal.
 14. The invention as recited in claim 13, wherein, step (a) comprises the steps of: (a1) detecting the presence or absence of the DC shift in the signal; and (a2) modifying the equalization based on the detected presence or absence of the DC shift in the signal.
 15. The invention as recited in claim 14, wherein, for step (a2), the equalization is modified to Nyquist equalization when step (a1) detects the presence of the DC shift in the signal.
 16. The invention as recited in claim 13, wherein step (b) comprises the steps of: (b1) combining i) the signal before applying the equalization and ii) the signal after applying the equalization of step (a) into a combined signal; and (b2) conditioning detection of the data based on the combined signal.
 17. The invention as recited in claim 16, wherein step (b1) comprises the step of logic ORing i) the signal before applying the equalization and ii) the signal after applying the equalization to generate the combined signal.
 18. The invention as recited in claim 16, further comprising the step of (b3) detecting the data based on the combined signal.
 19. The invention as recited in claim 18, wherein for, step (b3), the data is servo data comprising servo address mark (SAM) data and Gray data, and wherein step (b3) detects the SAM data based on the combined signal.
 20. The invention as recited in claim 19, wherein step (b3) further includes the step of decoding the Gray data in the signal without equalization based on the detected SAM data.
 21. The invention as recited in claim 18, further comprising the step of selecting either the i) the signal before applying the equalization and ii) the signal after applying the equalization for decoding the Gray data.
 22. The invention as recited in claim 21, wherein either the i) the signal before applying the equalization and ii) the signal after applying the equalization is selected based on a detection metric.
 23. The invention as recited in claim 16, wherein, for step (a), the equalization applied is a 2 D filter, where “D” is a unit discrete time delay.
 24. The invention as recited in claim 13, wherein, for step (b), the data is servo data, and wherein step (b) further comprises the step of decoding the servo data.
 25. The invention as recited in claim 24, wherein, for step (b), the servo data includes servo address mark (SAM) data and Gray data, and wherein step (b) includes the steps of detecting the SAM data and decoding the Gray data based on the detected SAM data.
 26. The invention as recited in claim 13, wherein the method is embodied by a processor of an integrated circuit.
 27. The invention as recited in claim 26, wherein the method is implemented by the processor in a read channel component of either a magnetic recording system or an optical recording system.
 28. A computer-readable medium having stored thereon a plurality of instructions, the plurality of instructions including instructions which, when executed by a processor, cause the processor to implement a method for detecting data in a signal read from a channel, the method comprising the steps of: (a) applying, with an equalizer, equalization to the signal to account for a DC shift in the signal; and (b) detecting the data based on either the presence or absence of the DC shift in the signal. 